This invention relates to a variable capacity compressor for compressing refrigerant for use in air conditioning systems for automotive vehicles, and more particularly to a compressor of this kind which is capable of varying the capacity thereof in response to change in the suction pressure depending upon a thermal load on the air conditioning system.
As a conventional variable capacity compressor of this kind, a vane compressor is known, e.g., from Japanese Provisional Utility Model Publication (Kokai) No. 63-99005 proposed by the present assignee, which has a control valve device arranged in a passage which communicates with a suction chamber and a high pressure chamber having discharge pressure introduced thereinto, and being operable to open and close the passage in response to change in pressure within the suction chamber depending upon a thermal load on the air conditioning system, thereby controlling the delivery quantity or capacity of the compressor.
However, according to the conventional variable capacity compressor, when it is operating with a low thermal load on the air conditioning system, discharge pressure is low and the flow rate of refrigerant gas discharged from the compressor is correspondingly low, so that the loss of pressure of refrigerant gas flowing through a hose connecting between the outlet of an evaporator and the inlet of the compressor is small. However, when the compressor is operating under a high thermal load, the flow rate of refrigerant gas discharged from the compressor increases so that the above pressure loss increases.
Further, the conventional compressor controls its capacity in response to change in the suction pressure dependent upon a thermal load on the air conditioning system. In other words, it controls its capacity so as to bring the suction pressure to a predetermined constant value, which is usually set at approximately 1.9 to 2 kg/cm.sup.2, for example, in order to obtain the maximum cooling capacity of the air conditioning system at a high thermal load, as shown by the one-dot chain line (a) in FIG. 7. However, while the loss of pressure between the evaporator outlet and the compressor inlet is large at a high thermal load and accordingly the pressure of refrigerant gas at the outlet of the evaporator is much higher than that at the inlet of the compressor or suction pressure at a high thermal load, the above pressure loss is so small that the pressure of refrigerant gas at the outlet of the evaporator can be almost as low as the suction pressure of the compressor, i.e., approximately 1.9 to 2 kg/cm.sup.2, which is lower than pressure at which freeze-up takes places in the evaporator, as shown by the broken line (b) in FIG. 7, thus causing the outlet of the evaporator to freeze up.